1. Field of the Invention
The present invention belongs to the technical field of an active matrix driven electro-optical device and a method for manufacturing the same, and more particularly, to an electro-optical device, which has storage capacitor electrodes for adding storage capacitance to pixel electrodes and which also has a conductive layer designated as a barrier layer for allowing favorable electrical conductance between a pixel electrode and a pixel switching thin film transistor (hereunder sometimes referred to as an TFT).
2. Description of Related Art
In a conventional active matrix TFT-driven electro-optical device, many scanning lines arranged along the columns of pixel electrodes, data lines arranged along the rows of pixel electrodes, and TFTs respectively corresponding to and disposed at intersections of the scanning and data lines are provided on a TFT array substrate. Each of the TFTs has a gate electrode connected to the scanning line, and also has a semiconductor layer, whose source and drain regions are respectively connected to the data line and the pixel electrode. Incidentally, especially, each of the pixel electrodes is connected to the drain region of the semiconductor layer of a corresponding TFT through a contact hole bored in an interlayer insulating film, because the pixel electrodes are provided on various kinds of layers of the TFTs and on the interlayer insulating film for insulating the pixel electrodes from one another. Further, when a scanning signal is supplied to the gate electrode of the TFT through the scanning line, the TFT is turned on. Moreover, an image signal supplied to the source region of the semiconductor layer through the data line is fed to the pixel electrode through the source-drain path of the TFT. Such supply of the image signal is supplied to each of the pixel electrodes through a corresponding one of the TFTs only for an extremely short time. Thus, generally, a storage capacitor is formed in each of the pixel electrodes in parallel with a liquid crystal capacitor so as to hold the voltage of an image signal, which is supplied through the TFT turned on for an extremely short time, for a time that is far longer than the extremely short time. On the other hand, in this kind of an electro-optical device, the source and drain regions and the channel region, which is formed between the source and drain regions, of each of the pixel switching TFTs are constituted by the semiconductor layers formed on the TFT array substrate. The pixel electrodes need to be connected to the drain regions of the semiconductor layers through a laminated structure containing wirings, such as the scanning lines, the storage capacitor lines, and the data lines, and a plurality of interlayer insulating films for electrically insulating these wirings. Incidentally, especially, in the case of staggered type or coplanar type polysilicon TFTs, each having a top gate structure in which a gate electrode is provided on a semiconductor layer, the interlayer distance from the semiconductor layer of the laminated structure to the pixel electrode is long, for example, about 1000 nm or more. It is, thus, difficult to form a contact hole for electrically connecting both the semiconductor layer and the pixel electrode. More specifically, it is extremely difficult to form such a deep hole only by dry etching, because of etching accuracy degradation that is caused by increasing the depth of a portion, on which the etching is performed, and that results in possibility of penetrating a target semiconductor layer to thereby form a hole. Thus, a combination of dry etching and wet etching is performed. However, this wet etching results in an increase in the diameter of the contact hole. Consequently, it is difficult to lay out necessary amounts of wires and electrodes in a limited region on the substrate.
Hence, recently, the following technique has been developed. That is, when the electrical connection between the data line and the source region is provided by making a contact hole, which is led to the source region of the semiconductor layer, in the interlayer insulating film formed on the scanning line, a relaying conductive layer designated as a barrier layer is formed from the same layer as that constituting the data line on the interlayer insulating film by forming a contact hole led to the drain region of the semiconductor layer. Then, a contact hole guided to this barrier layer from the pixel electrode is formed in the interlayer insulating film formed on the data line and this barrier layer. Such a configuration, in which the electrical connection from the pixel electrode to the drain region is provided via the barrier layer constituted by the same layer as that of constituting the data line, a contact hole forming step is facilitated, as compared with the case of forming a contact hole in such a way as to be led from the pixel electrode directly to the semiconductor layer. Moreover, the diameter of the contact hole led to the drain region via the barrier layer is smaller than that of the contact hole led directly to the semiconductor layer.
In the case of such a conventional electro-optical device, there is a keen ordinary demand for enhancing the picture quality of a displayed image. It is extremely important in meeting such a demand to realize a highly fine image display region or a fine pixel pitch, and to attain a high pixel aperture ratio (that is, to enhance the ratio of a pixel aperture region, which transmits display light, to a non-pixel-aperture region, which does not transmit display light, corresponding to each pixel).
However, this kind of conventional electro-optical device has a drawback in that the pixel aperture ratio is low at a highly fine pixel pitch because there is an essential limit to the degree of fineness of each of the electrode size, wire width, and contact-hole diameter, which results from fabrication techniques, and that thus, the proportion of the area of a region which contains such wires and electrodes relative to the area of the image display region increases.
Furthermore, when the degree of fineness of the pixel pitch is enhanced in this manner, it is difficult to realize the aforementioned storage capacitor which has sufficient storage capacitance and is incorporated into the limited region on the substrate. Incidentally, particularly, according to the aforementioned technique using the barrier layer, the barrier layer is constituted by conductive film, such as Al (aluminum) film, which is the same as the conductive film constituting the data line. Thus, the degree of flexibility of forming a contact hole is low owing to the position and material of the barrier layer. Moreover, it is extremely difficult to use the barrier layer for purposes other than that of performing the relaying function. Particularly, it is impossible to simplify the configuration of the device and to enhance efficiency of a manufacturing process by making the most of each of the layers of the fine laminated structure. Additionally, according to this technique, a chemical reaction is caused by bringing the Al film constituting the barrier layer in contact with an ITO (Indium Tin Oxide) film constituting the pixel electrode. As a result, the readily ionizable Al film is subject to corrosion. This impairs the electrical connection between the barrier layer and the pixel electrode. Thus, it is necessary to use a high melting point metallic film, for instance, Ti (titanium) film, as a second barrier layer, so as to provide the favorable electrical connection between the ITO film and the second barrier layer. Consequently, this conventional electro-optical device has a drawback in that the structure of the layers and the process of fabricating the layers are complexed.
The present invention is accomplished in view of the aforementioned drawbacks. A problem to be solved by the present invention is to provide an electro-optical device, which can favorably electrically relay between the pixel electrode and the thin film transistor and increase the storage capacitance by using a relatively simple configuration even when a fine pixel pitch is employed, and which also can display a high-picture-quality image, and to provide a method for manufacturing the electro-optical device.